1. Field of the Invention
This invention relates to devices for altering the area of a rocket nozzle and thus controlling thrust while the rocket is in flight.
2. Description of the Prior Art
It is known that it is sometimes desirable to adjust the exhaust area of the nozzle of a rocket which utilizes solid propellant during flight. Adjustment of the exhaust area provides control over thrust which cannot be accomplished by merely varying the amount of propellant burned in a solid rocket as it can in a liquid or hybrid rocket. Decreasing the exhaust area increases pressure within the motor and thus boosts thrust.
Variable-area rocket nozzles which permit thrust change substantially throughout the flight of the rocket are known. Such nozzles are very sophisticated. However, thrust control throughout the entire flight of a rocket is not always necessary. Also, the sophisticated variable-area nozzles have a drawback in that they utilize complicated and delicate pressure sensing, hydraulic and electrical parts and such parts are subject to frequent failure.
Since continuous thrust control is often not necessary and variable-area nozzles have drawbacks, research has been conducted on simplified two area rocket nozzles in which a relatively large throat area is provided at launch time and in which the throat area is decreased some time during flight to provide increased thrust.
In the most closely related device for decreasing the exhaust area of a solid rocket nozzle during flight known to the inventor, a housing containing a piston held pintle is suspended within the combustion chamber of the rocket just in front of the nozzle throat. The piston which holds the pintle is held in position by a shear pin at launch and retains the pintle out of (forward from) the nozzle throat. During flight, a gas generator produces gas which presses rearwardly on the piston, causes the shear pin to shear and forces the piston rearwardly. When the piston moves rearwardly, the pintle, which is held by the piston, moves into the throat of the nozzle and decreases the exhaust area.
The above-described device has two primary drawbacks. First, shear pins are somewhat unreliable in that they must be strong enough to retain when they are supposed to retain (e.g. retaining the pintle should the motor be inadvertently dropped in handling) and weak enough to shear when they are supposed to shear and this delicate balance between strength and weakness is hard to obtain even in this day and age of sophisticated metallurgy. Second, the above-described device relies on several O-rings to prevent exhaust gas from the burning propellant from entering the housing in which the piston held pintle is retained. One of the O-rings in particular is to prevent exhaust gas from leaking into the housing and exerting rearward pressure on the forward end of the pintle while the pintle is being retained forward. On the other hand, the pintle is designed in such a way that, once it has been released and forced rearwardly to decrease the exhaust area, exhaust gas is encouraged to leak into the housing, exert rearward pressure on its forward end, and retain it in its rearward position. At this time, the particular O-ring in question is bypassed by the exhaust gas due to the construction of the device. At any rate, a failure of the O-ring in question while it is supposed to be keeping gas out of the housing can result in premature shearing of the shear pin due to undesirable rearward pressure being placed on the forward end of the pintle.
It is the objective of this invention to overcome the drawbacks of the above-described device.